PCIs of BSs, which are used as identification information of the physical layer, are one of the configuration parameters requisite for radio cells that are set at the initial installation of the BSs. A PCI is a parameter in which a UE (User Equipment) identifies a cell and serves as information such that the UE acquires the synchronization of a BS.
The PCI includes a unique combination of one orthogonal sequence and one pseudorandom sequence. Because only a limited number of 504 PCIs are supported, the same PCIs are necessarily reused in other cells. Further, BSs that are newly installed need to select the PCIs for their cells.
Both conditions of being collision-free and confusion-free should be satisfied in order to achieve a PCI assignment. The collision-free and confusion-free conditions are defined by the 3GPP (3rd Generation Partnership Project) 36.902 as follows:                Collision-free: a PCI is unique in the area that the cell covers.        Confusion-free: a cell shall not have neighboring cells with identical PCI.        
Additionally, in recent years, small BSs to provide communication services with one to four subscribers, for example, femto cells that are private BSs, are installed indoors. The small BSs are located within the area of a macro BS with a wide radius in an outdoor space. In particular, a large number of small BSs exist in urban environments.
Further, the small BS is allowed to turn-on/off electrical power by a user's request. However, if the small BS is turned-on again, the small BS should be re-assigned with a new PCI and configuration information.
Therefore, in order to accommodate the above-described property of the small BSs and manage large numbers of small BSs, it is essential that an operation server, such as HeMS system, including SON (Self-Organizing Networks) functionality, plays a key role.